1. Field of the Invention
The present invention relates to the high-intensity focused ultrasound (HIFU) field for magnetic resonance imaging (MRI) monitoring, and more particularly to a method for reducing temperature measurement errors of the HIFU device for MRI monitoring.
2. Description of the Prior Art
The Magnetic Resonance (MR) Proton Resonance Frequency (PRF) temperature measurement method can be used to monitor the temperature changes of the HIFU heating part in the HIFU device for MRI monitoring, and its principle is to use the phenomenon that the resonance frequency of protons in water will drift as the temperature changes. The PRF temperature measurement method requires the generation of one benchmark image (MR phase diagram), also known as reference image, before the heating process starts. This reference image provides reference phase information, used to perform subtraction from the phase diagram (also known as heating image) obtained during or after the heating process to determine the exact value of the temperature rise in the heating area.
However, in the actual heating process, the position of the ultrasonic transducer (i.e. treating head) may change after collecting one reference image, and the magnetic susceptibility changes caused by the movement of the ultrasonic transducer will result in static magnetic field changes in the focus area of the ultrasonic transducer, causing the heating image subtracted from the reference image to generate an additional phase difference, leading to temperature measurement errors.
Currently, there are two common solutions for reducing temperature errors. One of the solutions can be referred to as the single reference image method. This method limits the movement range of the ultrasonic transducer after collecting the reference image, thereby limiting the temperature errors to an acceptable range. However, the space usage range of one reference image is very small, and the ultrasonic transducer will move in a larger space range during the HIFU treatment process. To measure the temperatures of individual focus positions of the ultrasonic transducer, it is required that the reference images at different positions be collected, which occurs frequently, thus increasing the complexity level of the temperature measurement and the entire treatment time.
The other solution for reducing temperature errors can be referred to as the self reference method, wherein the reference phase diagram is obtained without heating or collecting reference images but by means of an adapted extrapolation method by using the heating image itself. This method can only monitor the temperature changes around the HIFU focus, and it is difficult to monitor the temperature changes outside the focus in actual applications. In addition, the accuracy of adapted extrapolation is related to the complexity level of the phase diagram and the size of the heating area, so it is difficult to obtain stable, consistent and accurate results in the application.
Moreover, in Chinese patent application No. 200910004957.2, corresponding to U.S. Pat. No. 8,491,614, a method is disclosed for reducing MR temperature measurement errors, which includes: obtaining an MR phase diagram as a reference image before the HIFU device heats the heating area; obtaining another MR phase diagram as a heating image during or after the heating process of the HIFU device; calculating the temperature changes in the heating area according to said heating image and reference image; and which further includes: compensating for said temperature changes according to the magnetic field changes caused by the position changes of the ultrasonic transducer of the HIFU device.